Refer to enum discriminant initializers

The interesting thing about the example in the `items.enum...generics`
rule is that it shows that the generic parameters from the enclosing
enum can't appear anywhere within the initializer of the enum
discriminant.  We don't use "initializer" anywhere on this page yet,
but it is the correct word to use here, so let's use it.

Author: traviscross

src/items/enumerations.md

@@ -197,7 +197,7 @@ enum OverflowingDiscriminantError2 {
 ```
 
 r[items.enum.discriminant.restrictions.generics]
-Explicit enum discriminants may not use generic parameters from the enclosing enum.
+Explicit enum discriminant initializers may not use generic parameters from the enclosing enum.
 
 ```rust,compile_fail
 #[repr(u32)]